1. Field of the Invention
The present invention relates to an ad-hoc network wireless communication system and a method thereof, and more particularly, to an ad-hoc network wireless communication system and a method thereof that makes it possible to perform a reliable multi-hop ad-hoc communication if a MAC (Media Access Control) protocol based on a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) protocol is extended to the multi-hop.
2. Description of the Related Art
With the development of the hardware technology and the explosive increase of the spread and demand for portable terminals such as notebook computers, PDAs (Personal Digital Assistant), etc., there have been active attempts towards grafting the concept of wireless mobile communications on data communications based on the existing Internet protocol. A representative basic technology for this is an MP (Mobile IP) (hereinafter referred to as ‘mobile IP’).
Currently, a host, which uses a mobile IP in a wireless LAN (Local Area Network) environment, should perform a handoff in two of the OSI (Open System Interconnection) layers when it leaves a present cell and moves to a new cell. A handoff performed in a MAC layer is for securing a reliable wireless link in the new cell, and a mobile IP handoff occurring in the IP layer is for providing positional clarity of the host that makes it possible to continuously maintain service during the movement of the host without any change of the IP address.
A wireless LAN, which is a data communication system for providing mobility and scalability, facilitates its construction and management in comparison to the existing wire LAN, and currently provides a data transmission speed of 11 Mbps. Also, the mobile host on the wireless LAN can receive Internet service by connecting to the wire LAN at a high rate at any place without a cable.
The standard related to a physical layer and a data-link layer of the wireless LAN is described in the IEEE (Institute of Electrical and Electronics Engineers) 802.11. The wireless LAN is constructed by an ad-hoc network composed of a wireless terminal only or an infrastructure network connected to a wire LAN.
FIG. 1 is a view schematically illustrating an infrastructure network combined with a wire/wireless network. Referring to FIG. 1, the infrastructure network includes a BSS (Basic Service Set), an ESS (Extended Service Set), an AP (Access Point), a portal, and a DS (Distribution System). The AP is a bridge between a wire network and a wireless network, and connects the wireless host to the existing wire LAN such as an Ethernet. The mobile host can connect to the Internet through the portal. At this time, a cell is formed centering around the AP, and this is called the BSS. Several BSSs constitute the ESS, and the DS determines a forwarding path of packets to be transferred to the mobile host.
FIG. 2 is a view schematically illustrating an ad-hoc network.
In the wireless LAN, which is different from the wire network, the position of the host is changed at all times. Accordingly, the host, which leaves a present cell and moves to another cell, should determine a new AP in order to re-determine the communication link, and this process is called a “handoff” or “roaming” in the MAC layer. For a smooth handoff, the IEEE 802.11 standard provides techniques such as scanning, re-association and so on. Hereinafter, protocols in the ad-hoc network will be explained.
The mobile host, if the signal strength of the AP signal becomes lower than a specified value, searches for a new AP, and selects an AP that has the biggest signal. This process is called the scanning. If the AP is determined by the scanning method, the mobile host informs its existence to a new AP through the re-association process. Then, the AP informs the new position information of the mobile host to the DS, and the DS updates the position information of the host.
The wireless LAN adopts the CSMA (Carrier Sense Multiple Access) method that shares physical media in the same manner as the wire LAN. However, a collision may be frequently produced unlike the wire LAN. For example, in FIG. 2, although a node C is in a transmission range of a node B, it is outside a transmission range of a node A. Accordingly, while the node A transmits a message to the node B, the node C cannot sense the message transmitted from the node A to the node B, and thus the node C can transmit a message to the node B by accessing a channel to the node B. In this case, the message transmitted from the node C causes interference in receiving the message of the node B, and the node C becomes a hidden host or a hidden terminal of the node A. In order to solve this problem, many studies have been made for protocols such as a MACA (Medium Access Collision Avoidance) using a RTS/CTS (Request To Send/Clear To Send), a MACAW (MACA with Acknowledgement) obtained by improving the MACA using a selective control frame, a FAMA (Floor Acquisition Multiple Access) using both a non-persistent carrier sensing and the RTS/CTS, an IEEE 802.11 MAC DCF (Distributed Control Function) of a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) supporting the carrier sensing and the RTS/CTS, a DBTMA (Dual Busy Tone Multiple Access) that is a MAC protocol based on the RTS/CTS, etc.
FIG. 3 is a view explaining a MAC protocol. Referring to FIG. 3, the host that will transmit data first confirms whether the wireless link is in use. If the wireless link is in use, the host re-attempts confirmation after waiting a predetermined time. If it is confirmed that the wireless link is not in use, the host broadcasts a control message named ‘RTS (Request To Send)’ (No. 1). In the RTS message, an address of destination and a transmission time are recorded, and the neighboring hosts, which intend to use the wireless link through the RTS message, can estimate how long they will wait. Meanwhile, the host, which has received the RTS, transmits a CTS (Clear To Send) message to the transmitting host only in the case that there is not danger of collision (No. 2). The transmitting host, which has received the CTS, transmits data without fear of collision (No. 3), and waits for an ACK (acknowledgement) message for acknowledging receipt of the data (No. 4). If the transmitting host fails to receive the ACK message, it retransmits the data for a predetermined number of times until it succeeds.
In the MACA protocol, the hidden host can considerably be removed through the RTS/CTS packet change as described above. Due to this advantage, the IEEE 802.11 DCF mechanism has introduced the MACAW system and standardized the DCF.
FIG. 4 is a view illustrating the operational principle of the MACAW protocol. Referring to FIG. 4, in the case that a node A intends to communicate with a node D that is located outside a communication range, the node A transmits the RTS message to a node B that is located inside the communication range. The node B transmits the CTS in response to the received RTS. When the CTS message transmitted by the node B reaches the node A, the node A transmits packets to the node B in response to the received CTS message, and the node B transmits the ACK message in response to the received packets. After the ACK message is transmitted, a random back-off time is produced to avoid the collision on the network. Thereafter, the same process as above is performed from the node B to a node C, and from the node C to the node D, resulting in that the node A can communicate with the node D located outside the communication range.
Meanwhile, the MACAW is a mechanism optimized to the ad-hoc network based on a one-hop. Accordingly, as shown in FIG. 4, in the case that the mechanism is extended to a multi-hop, at least 2N RTS/CTS control packets and N ACK packets are required per N-hop communication. Also, a random back-off mechanism is required per hop. These control packets and the random back-off mechanisms cause a network overhead in a multi-hop environment, and increase an end-to-end delay.
Although the hidden hosts are considerably removed through the RTS/CTS packet exchange, the hidden hosts as illustrated in FIGS. 5A and 5B still remain in the multi-hop environment. That is, as shown in FIG. 5A, in the case that the node A intends to communicate with a node E located outside the communication range, it transmits the RTS message to the node B located within the communication range. The node B transmits the CTS message in response to the received RTS message. At this time, the CTS message transmitted by the node B is also transferred to the node C located within the communication range of the node B.
If the node C transmits the RTS message in order to communicate with the node D, the RTS message transmitted by the node C would also be transferred to the node B. In this case, the node C may not receive the CTS message transferred from the node B to the node C due to the transmission of the RTS message transmitted by itself. Also, a mutual collision may occur between the data packets transmitted from the node A to the node B and the data packets transmitted by the node C.
Also, as shown in FIG. 5B, in the case that the RTS message is transmitted from the node A to the node B and the RTS message is transmitted from the node D to the node C, a mutual collision may occur between the CTS message transmitted by the node B and the RTS message transmitted from the node D. Also, a mutual collision may occur between the data packets transmitted from the node A to the node B and the CTS message transmitted by the node C.
In order to solve the above-described problems, the MACA-BI has proposed a receiver initiated handshaking system. In this system, the transmission of a transmitting node is initiated in a manner that a receiving node transmits RTR (Ready To Receive) packets to the transmitting node without using the RTS control packets. The MACA-BI can reduce the number of RTS/CTS packets required for the N-hop communication to N at minimum. Also, the MACA-BI supports the RTR transmission of the receiving node by piggybacking the traffic generation characteristics owned by itself to the data packets. However, since the MACA-BI should perform a channel access through the RTS with respect to the node that initially performs the channel access, it requires the same system as the MACA. Also, since the MACA-BI uses the RTR transmission mechanism based on a traffic generating history of the transmitting node, it takes a scheduling system that is difficult to be practically implemented.
In order to solve the problems of the MACA-BI as described above, a MARCH protocol has been proposed. The operation principle of the MARCH protocol is illustrated in FIG. 6. Referring to FIG. 6, the first packet-transmitting node of the MARCH protocol informs a data transmission to the receiving node through an RTS control packet. Then, the receiving node informs the traffic initiation time point through the transmission of a CTS packet to a receiving node of the next hop as it transmits a confirmation packet of the RTS to the transmitting node through the CTS control packet. Through the above-described process, the MARCH protocol can reduce the number of RTS/CTS packets required for the N-hop communication to N+1 at minimum, and propose a design that can be practically implemented as well.
However, the MARCH protocol has the drawbacks in that it still has the same problems of hidden host as involved in the MACAW protocol, and causes the problems of new hidden host as shown in FIG. 7. That is, in the case that the RTS message is transmitted from the node A to the node B and the RTS message is transmitted from the node D to the node E, a mutual collision may occur between the CTS message outputted from the node B and the data packet outputted from the node D, and in this case, the node C becomes unable to output the CTS message, resulting in that a random back-off mechanism is required after the timeout process.
Also, in the multi-hop network environment, the MARCH protocol may cause a collision between the data and the CTS due to the transmission time difference between the nodes on two paths of the different connections performed according to the mechanism of the MARCH protocol as shown in FIG. 8.
FIG. 9 is a view illustrating a CTS blocking problem caused in the MARCH protocol. As shown in FIG. 9, in the case that communications are performed in the different connections according to the MARCH mechanism, the node C, after receiving the CTS message of the node B, disregards the RTS message of the node D by changing its mode to a standby mode at a time point that it intends to transmit the CTS message to the node B. Thereafter, the node C may be unable to transmit the CTS message due to the data packet transmission of the node D at a time point that it intends to transmit the CTS message, and the random back-off mechanism is required after the timeout process. Since this phenomenon is due to the collision occurring between the CTS packet and the data packet that is relatively longer than the CTS message unlike the MACAW, its probability of occurrence becomes greater than that of the hidden host of the MACAW.
As described above, the MARCH mechanism is liable to greatly heighten the probability of occurrence of hidden host while it reduces the number of RTS/CTS control packets.